Having previously shown that extending the photoperiod (day length) with bright artificial light was beneficial to patients with winter depression, we set out to: (1) replicate the finding, (2) examine whether it was mediated by sleep depriving the patients, and (3) investigate how other types of manipulation of environmental light might affect patients with SAD. In six inpatients and seven outpatients we replicated the finding that bright (2500 lux) full-spectrum light has antidepressant effects whereas dim (300 lux or less) light does not. By carefully monitoring wakefulness we established that this effect is not mediated via sleep deprivation (a known antidepressant treatment modality). In an uncontrolled study on eight patients, we showed that bright light in the evening alone had antidepressant effects in six, suggesting that the morning hours are not critical for effective treatment, as may be the case with sleep deprivation. In order to clarify the way in which changes in photoperiod influence mood, we exposed two SAD patients to an artifically shortened day length and later to dim environmental light for two one-week periods in an isolated environment during the spring months when they were euthymic. Neither of these conditions induced depression in either patient, contrary to our predictions. It is possible that SAD patients may be refractory to photoperiodic manipulations at certain times of the year, a phenomenon seen in certain animals. Further studies to elucidate the mechanisms involved in causing and reversing winter depressions are being planned. These include further photoperiodic manipulations, as well as the administration to patients of the hormone melatonin, which serves as a chemical transducer of darkness in animals.